It is known that antenna performance is dependent on the size, shape and material composition of the antenna elements, the interaction between elements and the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and a wavelength of the signal received or transmitted by the antenna. These physical and electrical characteristics determine several antenna operational parameters, including input impedance, gain, directivity, signal polarization, resonant frequency, bandwidth and radiation pattern. Since the antenna is an integral element of a signal receive and transmit path of a communications device, antenna performance directly affects device performance.
Generally, an operable antenna should have a minimum physical antenna dimension on the order of a half wavelength (or a multiple thereof) of the operating frequency to limit energy dissipated in resistive losses and maximize transmitted or received energy. Due to the effect of a ground plane image, a quarter wavelength antenna (or odd integer multiples thereof) operative above a ground plane exhibits properties similar to a half wavelength antenna.
Communications device product designers prefer an efficient antenna that is capable of wide bandwidth and/or multiple frequency band operation, electrically matched (e.g., impedance matched) to the transmitting and receiving components of the communications system and operable in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns). They also prefer a physically small antenna.
Consumer communications devices or devices incorporating a communications component, such as portable notebook computers, include antennas for various wireless communications services such as WLAN, WiMAX and cellular services. Due to the requirements for form and functionality, the physical space available for the antenna(s) is typically limited to narrow spaces close to and/or between conductive objects. But conventional antenna design approaches, such as PIFA-type antennas, work poorly in circumstances where the antenna is disposed in a narrow opening or gap (e.g., less than about 1/10 wavelength) between conductive objects. For example when the antenna is to be mounted between the display and keyboard portions of a notebook computer. Large areas of the screen and the keyboard are made from conductive metal, and the space between the two is effectively a long narrow gap between large conductive bodies. It appears that the geometric constraints of this antenna location allow effective propagation of only those modes with electric-field polarization across the gap (e.g., across the smaller dimension of the gap or between an edge of the screen and an adjacent edge of the keyboard). Commonly used antennas that work well in unbounded conditions, such as PIFA type antennas, may perform poorly when installed in the aforementioned gap location because of the electromagnetic constraints of the gap.
A slot antenna may consist of a conductive surface, usually a flat plate, with a hole or slot formed in the plate. The slot may be fed by connecting antenna feed conductors across the slot. For example, a coaxial cable shield is connected to a first edge of the slot (or bonded to the plate) while a center conductor is connected to a second slot edge (parallel to the first edge). Supplying a driving frequency between the coaxial cable shield and the center conductor, causes the slot antenna to radiate electromagnetic waves similar to a dipole antenna. The shape and size of the slot and the driving frequency determine the radiation pattern.
Slotted cylindrical antennas are known as first described by Andrew Alford in 1946 and discussed by John D. Kraus in Antennas: For all Applications, third edition 2002. The antenna comprises a hollow conductive cylinder with a single narrow rectangular slot formed therein. Generally the slot is longer than λ/2 at the operating frequency of the antenna. An antenna feed is connected across the small dimension of the slot (identical to the feed arrangement for a conventional slot antenna). In the Kraus description of slotted antennas, the cylinder is shown as a true circular cylinder, however in other references the term cylinder is applied to other cross-section shapes such as a rectangular cross section.
The impedance of the path around the circumference of the cylinder is sufficiently low so that most of the current tends to flow in horizontal loops around the cylinder. If the diameter of the cylinder is a sufficiently small fraction of a wavelength, for example less than about λ/8, an upright cylinder with a vertical slot radiates a horizontally polarized field with a radiation pattern that is substantially circular in the horizontal plane. As the cylinder diameter increases, the pattern in the horizontal plane tends to become more unidirectional with the maximum radiation from the side of the cylinder where the slot is located.